All three T. brucei subspecies constitute a significant health and economic problem in sub-Saharan Africa. Although T. brucei brucei does not infect humans, it is an effective cattle pathogen that virtually prevents the raising of livestock in endemic areas. T.b. brucei does not infect humans because it is killed by non-immune trypanosome lytic factors (TLFs) in human serum, while the human resistant T. brucei subspecies are resistant to these TLFs. Two distinct TLFs, called TLF1 and TLF2, are known to exist. Several lines of evidence suggest that trypanosomal receptors mediate the uptake of TLFs and subsequent trypanolysis. The specific aims of this proposal are to 1) evaluate whether both serum-sensitive and serum-resistant trypanosomes express these receptors and show similar intracellular routing of TLFs, 2) characterize the binding parameters and mode of membrane attachment for the TLF receptor(s) in order to purify and eventually clone them, and 3) characterize the subcellular localization of the TLF receptor(s) and their distribution throughout the kinetoplastidae. Trypanosome receptors in general may constitute the Achilles heel of the parasite as they mediate the uptake of essential nutrients such as transferrin, LDL, and HDL from the host, and they are generally located in the flagellar pocket which is accessible to antibodies. Currently, these are the only known receptors in bloodstream form trypanosomes, and only the transferrin receptor has been cloned. The identification of receptor(s) for TLF could provide another example of an invariant trypanosome surface protein, which is likely to be localized to the flagellar pocket. It has been proposed that a combination of such receptors could provide the basis for a multicomponent vaccine for either humans or livestock.